1. Field of Invention
The present invention relates to a nickel-zinc battery, and more particularly to an additive for nickel-zinc battery to increase the efficiency and lifespan of the battery.
2. Description of Related Arts
At present, the growing environmental situation forced many countries to issue strict environmental regulations, green and low-carbon economy has become a trend. As the oil price remains high, the prosperity of internet and electronic products have given rise to new growing markets of rechargeable batteries. Particularly, the fast growth of hybrid electric vehicle (HEV), plug-in hybrid electric vehicle (PHEV), and electric vehicle (EV) market have led to urgent need of a kind of battery that is of higher energy, higher power, more stable, safer, and more environmental friendly. Conventional battery technologies, such as lead acid and nickel-cadmium batteries, cannot meet the market needs. In addition, these batteries are not in line with the requirements of environmental protection. Lithium batteries, though very successful in the portable electronic applications, cannot meet the requirements of large systems due to inadequate power, high price, and risk of safety.
The emerging nickel-zinc (Ni—Zn) battery technology has the potential to fulfill various application needs. The nickel-zinc is a rechargeable battery with high power and adequate energy while pollution level, risk level and cost are low since heavy metals such as Pb, Cd, and Hg are not used in the manufacture and the battery is non-flammable.
Despite their advantages, Ni—Zn batteries have unresolved problem of short cycle life for many years. The reason of its short cycle life is believed to be caused by: (1) zinc dendrites growth during charging process which causes short circuit in the battery and limits its service life; (2) zinc which is soluble in alkaline electrolyte and does not keep in the same place during charging and discharging processes for causing shape change; and (3) electrode material falling during cycling leading to loss in cell capacity.
Most of the researches on the Ni—Zn batteries are focused on the prevention of dendrites and/or distortion of Zn anodes, but fewer researches are aimed at the cathodes. The importance of the cathode composition is much neglected. As a common method, the Ni cathode of a Ni-MH or Ni—Cd battery is completely employed in the Ni—Zn batteries, leading to poor performance.
The charging efficiency of nickel cathode electrode is low in a nickel-zinc battery, especially at the late charging stage. Several problems can be caused by the cathode when it is at the low charging efficiency stage. First, the zinc anode could be overcharged and dendrites are easily built. Second, unwanted and extra oxygen evolution may occur at the cathode during charging. Third, the cathode may expand and get loosely packed when overcharged. Fourth, the zinc anode has to be over-weighted to counter balance the low charging efficiency of the cathode.
In the conventional arts, cobalt and cobalt oxides or hydroxides have been applied in the cathode for increasing the charging efficiency, promoting the overpotential of oxygen evolution, and intensifying the depth of discharging. In theory, the metal cobalt cannot be dissolved in alkali. However, the cobalt in the surface is unavoidably oxidized. Then, the cobalt oxides and its hydroxides can both be dissolved in the alkali liquid, imposing safety concern to the battery.
When cobalt compounds dissolved within the alkali electrolyte, it will contact with the Zn electrode. Then the cobalt compounds will promote hydrogen evolution at the Zn electrode, resulting in great risk concerning safety and making the battery fail to meet the requirements of application. Therefore, it is apparent that the cobalt additive is not a solution for a Ni—Zn battery to increase the charging efficiency, promote the overpotential of oxygen evolution, intensify the depth of discharging, while maintaining the high power capability and environmental friendliness of the nickel-zinc battery.